1. Field of the Invention
The present invention relates to diesel engine lubricating oils and to the control of soot induced viscosity increase of the lubricating used in such engines.
2. Description of the Related Art
Internal combustion engines function by the combustion of fuels which in turn generate the power needed to propel vehicles. In the case of a diesel engine, the fuel is a diesel fuel and the combustion thereof generally results in emissions from the exhausts of such vehicles which comprise three main components. These are: soot and particulate matter, carbon monoxide and nitrogen oxides (the latter will hereafter be abbreviated as NOx for convenience). To alleviate environmental concerns, research is ongoing to reduce the levels of these emissions. NOx emission can be reduced by lowering the temperature at which the fuel is combusted in the engine. Typically this is achieved by retarding the combustion, i.e., by injecting the fuel shortly after the peak temperature is reached in the cylinder. However, this retarded combustion has the disadvantage that it causes more soot to accumulate in the engine lubricant partly due to incomplete combustion of the fuel because of the lower combustion temperature, and partly due to increased soot deposition on the cylinder wall which is drawn down into the lubricant with the downward stroke of the piston. The presence of soot in the lubricant has the adverse affects of causing viscosity increase and accelerated wear. It is important that soot induced viscosity increase be controlled such that the lubricant stays within viscosity grade in order to maintain its expected performance and to enable quick and clean drainage of the engine during servicing.
The formation of soot may be alleviated to a significant extent by operating the diesel engine at relatively higher temperatures. However, the higher temperatures whilst mitigating the formation of soot also result in the formation of increased amounts of NOx. If, however, the engine temperature is towered, incomplete combustion ensues and whilst this reduces the amount of NOx formed in the emissions, it also substantially increases the amount of soot generated. The soot so formed can manifest itself in two ways. It can either appear as a thick black smoke emitted from the exhaust of the vehicle or can be accumulated in the engine lubricant. As the soot builds up in the lubricant, the latter becomes more and more viscous and upon reaching a critical value can cause gelation of the lubricant and may eventually cause seizure of the engine.
Several methods have been put forward to alleviate this problem including the use of one or more of dispersants, metal salts and solvents which may be ethers, esters and the like. The dispersants function by forming a coating of the dispersant on the surface of soot particles and thereby minimizing the tendency of the soot particles to agglomerate. However, the potency of the dispersants to perform this function, in turn, declines with time and thus, one of the methods of improving the useful life of lubricants, particularly crankcase lubricants, would be to improve the dispersancy retention capability of crankcase lubricants. This may be achieved, e.g., by minimizing the risk of oxidation of the dispersants under the conditions prevalent in the engines during use. One such method is described in U.S. Pat. No. 5,837,657 which discloses a method of improving the performance of a sooted diesel oil and controlling soot induced viscosity increase by adding to the diesel oil a minor amount of a trinuclear molybdenum compound of the generic formula MO3SkLnQz wherein L is a ligand having organo groups, n is from 1 to 4, k various from 4 through 10, Q is a neutral electron donating compound such as, e.g., water, amines, alcohols, phosphines and ethers, and z ranges from 0 to 5.
Hydrocarbon base oils have differing solvency characteristics that affect their capability to solubilize performance additives. Highly paraffinic hydrocarbon base oils (those having low levels of aromaticity) are known to have low-to-poor additive solubility characteristics. For example, such low-solvency hydrocarbon base oils include polyalpha olefins (PAO) which are 100% isoparaffinic and have essentially 0% aromatics content. Similarly, wax isomerate base oils, in particular hydroisomerized Fischer-Tropsch (F-T) waxes, often called Gas-to-Liquids (GTL) lubricant base oils, are very highly paraffinic and have essentially 0% aromatics content. The base stock is derived from a waxy, F-T synthesized hydrocarbon feed fraction comprising hydrocarbons having an initial b.p. in the range of approximately 650-750° F., by a process which comprises hydroisomerizing the feed and optionally dewaxing the isomerate. The lubricant also contains hydrocarbonaceous and synthetic base stock material in mixture with the F-T derived base stock. Consequently, such wax isomerate base oils would be expected to have low solvency and poor additive solubility performance.
High isoparaffinic base stocks, however, are advantageous in soot control for diesel engine lubricants. Lower soot-induced viscosity increase and lower soot-induced wear are observed for diesel engine lubricants with higher saturate contents. In addition, GTL base oils are essentially sulfur-free, which is highly desirable for the next generation engine lubricants such as GF-5 and PC-10. In these new engine lubricant categories, a maximum sulfur level is defined for improved compatibility with new low emission engines equipped with aftertreatment devices.
It would be advantageous if a way could be found to reduce the soot induced viscosity increase experienced in diesel engine lubricating oils during use while not negatively affecting the desired viscosity modifying effect of polymeric viscosity modifier which are normally added to diesel engine lubricating oils without the need of employing cosolvents.